Because of their high dielectric constants, low dissipation factors, high insulation resistance times capacitance products and stable temperature coefficients, the ceramic compositions of this invention are useful in manufacturing multilayer ceramic capacitors (hereinafter MLC) which require a high capacitance and which typically have a small physical size. MLC's are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder upon which conducting metal electrode layers, usually consisting of a palladium/silver alloy, are placed. The material is then densified by firing to form the MLC. Barium titanate (BaTiO.sub.3) is frequently used in the formation of MLC's due to its high dielectric constant. The stability of the dielectric constant of the MLC over a wide range of temperatures, however, and its insulation resistance, are also important factors to be considered in selecting ceramic compositions for use in MLC's. The electrical properties of dielectric ceramic compositions vary substantially with temperature increases or decreases. For example, the insulation resistance of ceramic compositions may vary substantially with grain size after final sintering.
It is known to produce a temperature stable MLC by firing BaTiO.sub.3 together with minor oxide additives for controlling the final dielectric properties. Ceramic compositions having dielectric constants between about 3000 to about 4700 at 25.degree. C. have been disclosed which have flat TC characteristics and in which the dielectric constant does not vary more than .+-.15 percent from the reference value at 25.degree. C. The dissipation factors of these known ceramic composition, however, are approximately 2.0% and their RC product is between about 3000 to about 4000 ohm-farads at 25.degree. C. Although these electric parameters meet the requirements described in most industrial specifications (e.g., EIA-RS198C, IEC-384-10, and JIS-RC-3698, which require an RC value of greater than 1000 ohm-farads at 25.degree. C. and higher than 100 ohm-farads at 125.degree. C.) it has been found that when the materials are used in large scale MLC manufacturing it is not possible to achieve such dissipation and RC factors.
In many cases, MLC's are further processed with solder coating, barrier layer plating, lead attachment and epoxy coating to produce finished products. These processes increase the dissipation factors and reduce the RC product. Finished MLC's made with the materials disclosed in the prior art often have electric properties which are much less favorable than those of the ceramic compositions from which they are made. These factors limit the production yields for MLC's and also increase the manufacturing cost per device.
The need exists, therefore, for a dielectric ceramic composition useful in MLC applications which evidences a stability of the dielectric constant over a wide temperature range. Ideally, the dielectric constant of such a ceramic composition would not change from its base value at 25.degree. C. by more than about .+-.15 percent over a temperature range of from -55.degree. C. to 125.degree. C. The RC product of such composition would preferably be more than 1000 ohm-farads at 25.degree. C. and more than 100 ohm-farads at maximum working temperature, which is 125.degree. C. in most cases.